(1) Field of the Invention
The present invention relates to a toner for developing statically charged images, which is used for the electrophotography and the like. More particularly, the present invention relates to a long-life toner for developing statically charged images, in which stable frictional charging characteristics are obtained without contamination of a frictional charging member such as a carrier.
(2) Description of the Related Art
A toner for developing statically charged images, which is used in the field of the electrophotography or the like, is composed of colored resin particles having a particle size of 5 to 20 .mu.m, which comprise a resin binder and, dispersed therein, a colorant, a charge-controlling dye and the like. A resin having a desired electroscopic property and a desired binding property, such as a styrene resin, is used as the resin binder. An organic or inorganic coloring pigment such as carbon black is used as the colorant. A nigrosine compound, a monoazo dye or a metal complex of salicylic acid or naphthoic acid is used as the charge-controlling dye. Most typically, this toner for developing statically charge images is prepared according to the so-called pulverization method comprising melt-kneading a resin binder with a colorant and a charge-controlling dye, cooling the kneaded composition, pulverizing the cooled solid product and classifying the pulverized product to adjust the particle size within a certain range. However, the yield of the toner thus obtained through pulverization and classification is very low, and large equipment is necessary for the production and the manufacturing cost of the toner is very large. Moreover, the toner particles obtained according to the pulverization method is irregular in the shape and hence, the flowability of the toner is generally low and blocking is often caused. Accordingly, for overcoming these defects of the toner having an indeterminate shape, which is provided according to the pulverization method, trials have been made to improve the flowability and frictional chargeability by preparing shperical toner particles. This spherical toner is prepared according to the spray-drying method comprising dissolving or dispersing a binder resin, a colorant, a charge-controlling dye and the like into an organic solvent such as toluene and spray-drying the resin solution or dispersion to obtain a toner, or the suspension polymerization method comprising suspending a polymerizable composition comprising a polymerizable monomer forming a binder resin, a colorant, a charge-controlling agent and the like into an aqueous dispersion medium and directly forming a toner by carrying out the polymerization reaction. This spherical toner has a higher flowability than the toner having an indeterminate shape, and the spherical toner shows a better stirrability in a stirrer and a better conveyability. Moreover, the spherical toner is advantageous in that the contact area among toner particle or the contact area between the toner and a frictional charging member such as a magnetic carrier (in case of a two-component type developer) is large, effective friction is performed among particles and because of a good flowability as well as these characteristics, frictional charging can be effected uniformly and efficiently.
However, as the result of investigations made by us, it was found that the spherical toner involves the following problem. Namely, in a toner consisting of particles rendered spherical to a certain level, the effective area for the contact and friction in the surfaces of the particles in large and the mobility of the toner particles is increased, and therefore, the frictional charging efficiency is increased and the rising characteristics at the charging are enhanced. However, the charge-controlling dye present on the surfaces of the particles are dropped by sharp frictional stirring, and the dropped charge-controlling dye stains the surface of the frictional charging member such as the carrier and gradually renders the frictional chargeability characteristics unstable. Therefore, as the copying operation is continued, image fogging or reduction of the image density is caused.